Atmospheric river

Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Beyond NCERT

Meaning

An atmospheric river (AR) is a long, narrow band of concentrated water vapour in the air that carries moist air from the tropics to higher latitudes [1]. It matters because it moves huge amounts of water through the sky. When it hits mountains, it can drop extreme rain or snow and cause floods and mudslides [1], including many of India's deadliest monsoon floods [3].

Explanation

How it forms

From warm sea to heavy rain on land [2]

• Warm tropical seas heat the air above them, so a lot of water evaporates (turns into water vapour).
• Strong low-level winds (winds close to the ground) gather this vapour into a narrow band and carry it poleward (towards the poles).
• Over cooler land, the vapour turns back into water and falls as rain or snow.

Size and scale

• Width: about 400–600 km (250–375 miles) [2].
• Length: it can be more than 1,600 km (1,000 miles) [2].
• Share of water moved: ARs carry more than 90% of the water vapour that moves from the tropics to the mid-latitudes [1].
• Strength: a strong AR carries as much water vapour as about 7.5–15 times the average flow of the Mississippi River at its mouth [1].
• So an AR is like a river, but it carries invisible vapour through the sky, not liquid water on the ground.

What happens when it hits mountains

Moist air meets a mountain wall [1]

• The mountain pushes the moist air upward.
• As the air rises, it cools, and its water vapour condenses (turns into tiny water drops).
• This gives extreme rain or snow on the side of the mountain that faces the wind (the windward side).
• The results are flash floods (sudden floods that come within hours) and mudslides.

Good and bad sides

• Useful: on the US west coast, about 30–50% of yearly precipitation (rain plus snow) falls in just a few AR events [1]. ARs refill water supplies there.
• Harmful: the same events can bring floods and mudslides when too much water falls too quickly [1].

Where it occurs

World

• California (US west coast): hit by the Pineapple Express, an AR that starts near Hawaii and moves towards California.
• US west coast in general: about 30–50% of yearly precipitation comes from a few ARs [1].
• Western Europe and Chile: floods there are also linked to ARs.
• General pattern: ARs mostly reach the west coasts of mid-latitude continents. This is also where the westerlies (mid-latitude winds blowing from the west) bring moist ocean air onto the land.

India

• A study of 1985–2020 found that about 65% of ARs reaching land in India were linked to floods [3].
• Seven of India's ten most severe summer-monsoon floods with heavy loss of life were linked to ARs [3].
• More than 95% of AR events in India happen in the summer monsoon (June–September) [3].
• Trend: over the past seven decades, moisture transport has grown stronger and low-level winds have strengthened. This has pushed AR-linked rainfall upward where ARs reach land in India [3]. Research on this is still going on.
• Areas at risk: the Western Ghats and the Himalaya. Here the mountains force moist monsoon air upward.

Don't confuse with

• Jet stream: a very fast, narrow wind high up in the atmosphere. It is a band of wind. An atmospheric river is a band of water vapour carried by low-level winds.
• Tropical cyclone: a round, spinning storm around a low-pressure centre. An AR is a long, narrow band, not a circular storm.
• ITCZ (Inter Tropical Convergence Zone): the low-pressure belt near the equator where the trade winds meet and air rises to give rain. It is fixed near the equator. An AR carries moisture away from the tropics towards higher latitudes.
• Pineapple Express: this is not a different thing. It is one named AR that runs from near Hawaii to California. Do not treat it as a separate kind of wind.

Prelims Hooks

• ARs carry more than 90% of the water vapour that moves from the tropics to the mid-latitudes [1].
• Pineapple Express → California. It starts near Hawaii.
• Typical AR size: 400–600 km wide and can be more than 1,600 km long [2].
• A strong AR carries 7.5–15 times the average flow of the Mississippi River at its mouth [1].
• In India, more than 95% of ARs come in the summer monsoon (June–September) [3]. Trap: they are not mainly a winter feature in India.
• 7 of India's 10 deadliest summer-monsoon floods were linked to ARs [3].

Mains Points

• ARs as a growing flood risk in India (GS-III, disaster management): about 65% of ARs that reached land in India during 1985–2020 were linked to floods, and 7 of the 10 deadliest monsoon floods were AR-linked [3]. Stronger moisture transport over seven decades is pushing AR rainfall upward [3]. India needs:
• AR-aware flood forecasting by IMD and CWC.
• Better early warnings for the Western Ghats and the Himalaya.

• Climate-proof infrastructure such as dams, roads and drainage.

• Two sides of ARs in water management (GS-I/GS-III): on the US west coast, a few ARs give 30–50% of yearly precipitation [1]. They are a key water source and also a flood and mudslide hazard. Regions that depend on ARs need storage to capture the water and warnings to prevent the damage.

• Link to the wider circulation (GS-I): ARs move tropical moisture poleward with the low-level winds. They hit mountains on mid-latitude west coasts (California, western Europe, Chile), and in India they hit hills during the monsoon. This shows how the planetary winds and relief (the height and shape of the land) together decide where extreme rain falls.

Related concepts

• Trade winds
• Southeast trade winds
• Westerlies
• Roaring Forties
• Polar easterlies

Read more

• The planetary winds: trades, westerlies and polar easterlies